1. Field of the Invention
The invention relates generally to apparatus and methods for creating foamed fracturing fluids carrying high concentrations of proppant material.
2. Description of the Prior Art
During the completion of an oil or gas well, or the like, one technique which is sometimes used to stimulate production is the fracturing of the subsurface producing formation. This is accomplished by pumping a fluid at a very high pressure and rate into the formation to hydraulically create a fracture extending from the well bore out into the formation. In many instances, a proppant material such as sand is included in the fracturing fluid, and subsequently deposited in the fracture to prop the fracture so that it remains open after fracturing pressure has been released from the formation.
In recent years, it has become popular to utilize a fracturing fluid which has been foamed. There are a number of advantages of foamed fracturing fluids which are at this point generally recognized.
One advantage of foamed fracturing fluids is that they have low fluid loss characteristics resulting in more efficient fracture treatments and reduced damage to water sensitive formations.
Also, formed fracturing fluids have a relatively low hydrostatic head thus minimizing fluid entry into the formation and its resulting damage.
The foamed fracturing fluids have a high effective viscosity permitting the creation of wider vertical fractures and horizontal fractures having greater area.
Also, foamed fracturing fluids typically have a high proppant carrying capacity allowing more proppant to be delivered to the site of the fracture and more proppant to remain suspended until the fracture heals.
Currently available foamed fracturing fluids do have at least one major disadvantage, and this pertains to proppant concentrations available with currently practiced foam generation techniques. Typically, current techniques involve blending a mixture of proppant and liquid containing a suitable surfactant. The mixture is pumped to high pressure after which the gaseous phase, typically nitrogen or carbon dioxide, is added to produce the foamed sand-laden fracturing fluid.
This technique involves an inherent proppant concentration limitation due to the concentration limitation of the proppant/liquid mixture. The theoretical maximum concentration of a sand/liquid mixture is approximately thirty-four pounds of sand per gallon of liquid. This corresponds to a liquid volume just sufficient to fill the void spaces of bulk sand. In common practice, this maximum is further limited by the blending and pumping equipment capabilities and lies in a range of 15 to 25 lb/gal.
Typically, foams are produced which have approximately three unit volumes of gaseous phase per unit volume of liquid phase corresponding to a foam quality, that is a gaseous volume fraction, of 75%. Herein lies the problem; when the liquid phase is foamed, the gas expands the carrier fluid to approximately four times its original volume. A sand concentration of 25 pounds of sand per gallon of liquid in a sand/liquid slurry is reduced to approximately six pounds of sand per gallon of carrier fluid, i.e., foam, by the process of foaming. Even the theoretical maximum sand concentration of 34 lb/gal in the sand/liquid slurry would only produce an 8.5 lb/gal concentration in a 75% quality foam.
The concentration of proppant in the fracturing fluid is of considerable importance since this determines the final propped thickness of the fracture. A fracturing fluid with a sand concentration of 34 pounds of sand per gallon of carrier fluid could theoretically prop the fracture at its hydraulically created width.
Another problem encountered with many fracturing fluids including foam also involves proppant concentration and this pertains to the fracturing fluid's compatibility with the formation core and formation fluids, particularly in gas wells. For example, many formulations contain clays which swell when contacted by water base fluids resulting in reduced formation permeability. Foamed fracturing fluids reduce this problem due to their low fluid loss and low hydrostatic head characteristics, both of which result in less fluid entering the formation. However, even with foamed fracturing fluids, the theoretical maximum sand concentration is 34 pounds of sand per gallon of liquid phase of the foam and as previously mentioned, the current practical limit is about 25 pounds per gallon. A foamed fracturing fluid with a greater concentration of sand to liquid would be highly desirable for water sensitive formations since a given amount of sand could be delivered to the formation with less liquid in the carrier fluid.
Prior to the present invention, the typical approach to these problems of the inherent limitation of sand concentration in foam, created by the limitations on the proportion of sand which can be carried by the liquid prior to foaming, has been to concentrate the sand in the sand-liquid slurry prior to foaming.
One example of a foam sand concentrator of that type which also generally explains the inherent limitations in the prior art foaming processes, is shown in U.S. Pat. No. 4,448,709 to Bullen. Bullen indicates that the physical limitation of the high pressure pumps utilized in his process limits the sand concentration in the initial liquid/sand slurry to about ten pounds of sand per gallon of liquid. When such a slurry is formed to a 75% quality, the resulting foam carries 21/2 pounds of sand per gallon of foam, if no concentration is used. The Bullen concentrator is stated to be capable of removing about 50% of the liquid from the slurry, thus doubling the proppant concentration in the subsequent foam to a maximum of about five pounds per gallon of 75% quality foam, that is twenty pounds per gallon of liquid in the resulting foam.
Other examples of devices which concentrate sand in the sand-liquid slurry prior to foaming are shown in U.S. Pat. No. 4,126,181 to Black and U.S. Pat. No. 4,354,552 to Zingg.
Thus it is apparent that although the prior art has recognized the problem of the inherent limitations on sand concentration in foamed proppant carrying fracturing fluids, no satisfactory solution to the problem has been provided prior to the present invention.